Cosmic Rays on a Plane!

I took an international flight recently, and did something I've intended
to do for some time: monitor the
background
radiation flux as the plane changed altitudes. I brought along a
QuartaRAD RADEX RD1706Geiger-Müller
counter which detects beta particles (high energy electrons) and
photons in the x-ray and gamma ray spectra and displays a smoothed
moving average of the radiation dose in
microsieverts (μSv)
per hour. The background radiation depends upon your local
environment: areas with
rocks such as granite
which are rich in mildly radioactive uranium and thorium will
have more background radiation than those with rocks such as
limestone.

One important component of background radiation is cosmic rays caused
by high energy particles striking the Earth's atmosphere. The
atmosphere is an effective radiation shield and absorbs many of these
particles before they reach sea level, but as you go to higher
altitudes, fewer particles are absorbed and you experience a higher
background radiation dose from cosmic rays. Background radiation at
sea level is usually around 0.10 to 0.13 μSv/h. At Fourmilab, at an
altitude of 806 metres above mean sea level, it usually runs
around 0.16 μSv/h.

I waited until the flight was at cruising altitude before turning on
the detector and placing it on my tray table near the window of my
window seat. This was not a high-flyer: the plane was a
Bombardier Q400
Dash 8 regional turboprop on a medium-range flight within Europe,
with a cruising altitude of 7000 metres (the plane's service ceiling
is 8229 metres, modest compared to the Boeing 747-8's ceiling of 13,000
m). My first reading was:

Wow! 1.24 microsieverts per hour is almost ten times the
usual reading near sea level. And this was inside the fuselage of an
airplane cruising at a modest altitude.

About half way through the flight, we encountered moderately high
turbulence (enough to turn on the seat belts sign, but nothing really
scary), and the pilot in command requested a lower altitude to try to
escape it. Air traffic control approved a descent to 6000 metres.
During the descent, the background radiation level smoothly decreased.
Here is part way down the slope.

And now we're at at the new cruising altitude of 6000 m.

Finally the plane began its descent for landing. Here are readings on
the way down, with the last one on final approach over water shortly
before touchdown on the runway on the coast.

Now the radiation level has fallen to that around sea level. But wait,
there's more!

This is at an altitude of just dozens of metres, still over water,
seconds before touchdown. Background radiation is now around half the
usual at sea level. (This wasn't a fluke—I got this reading on several
consecutive measurement cycles.) But think about it: the contribution
to background radiation from terrestrial sources (such as thorium and
uranium in rocks) and cosmic rays are about the same. But in an
airplane flying low over water, the terrestrial component is very small
(since the sea has very few radioactive nuclides), so it's plausible
that we'll see around half the background radiation in such a situation
as on terra firma. Indeed, after landing, the background
radiation while taxiing to the terminal went back up to around
0.13 μSv/h.

It would be interesting to repeat this experiment on an
intercontinental flight at higher altitude and through higher
latitudes, where the Earth's magnetic field provides less shielding
against cosmic rays. But the unpleasantness of such journeys deters me
from making them in anything less that the most exigent circumstances.
There is no original science to be done here: extensive monitoring and
analysis of the radiation dose experienced by airline passengers and
crews has been done. This is a Fourmilab “basement science” experiment
(well, not in the basement, but in a shrieking aluminium death tube)
you can do yourself for amusement. If you do this on a crowded flight,
your seatmate may inquire what're you're up to. “Measuring the cosmic
radiation dose we're receiving on this flight.” This can either lead
to a long and interesting conversation about atmospheric absorption of
cosmic rays, background radiation, and
radiation
hormesis or, more likely, your having an empty seat next to you for
the remainder of the flight. Think of it as win-win. There were only
seven passengers on this flight (I don't go to places that are too
crowded—nobody goes there), so this didn't come up during this
experiment.

Return Flight

A couple of weeks later, the return flight was on an
Embraer E190
regional turbofan airliner. The altitude of the flight
was never announced en route, but this aircraft has a
service ceiling of 12,000 m and usually cruises around
10,000 m, substantially higher than the turboprop I took
on the outbound flight. I expected to see a higher radiation
level on this flight, and I did.

Did I ever! Most of the readings I obtained during cruise
were around 3.8 μSv/h, more than thirty times typical
sea level background radiation. (I'd show you one of these
readings, but there was substantial turbulence on the flight
and all of my attempts to photograph the reading are blurred.)
During the cruise, I got several substantially higher values
such as the 5.07 μSv/h shown above—more than
forty times sea level.

Why was there such variation in background radiation during
the cruise? I have no idea. If I had to guess, it would
be that at the higher altitude there is more exposure to
air showers, which
might account for the greater variance than observed at
sea level or lower altitude in flight. Or, maybe the
gremlin on the wing
was wearing a radioactive bracelet.